There has been known an art of providing a particulate filter (hereinafter referred to simply as a filter in some cases), a selective reduction-type NOx catalyst (hereinafter referred to simply as an NOx catalyst in some cases), and the like as an exhaust gas control apparatus in an exhaust passage of an internal combustion engine. The filter collects a particulate matter (hereinafter referred to as a PM in some cases) in exhaust gas that is produced through the combustion of fuel in the internal combustion engine. The NOx catalyst reduces NOx in exhaust gas, using ammonia as a reducing agent.
In Patent Document 1, there is disclosed an art of providing a filter in an exhaust passage of an internal combustion engine and further providing a PM sensor in the exhaust passage downstream of the filter. Besides, it is described in Patent Document 1 that an NOx catalyst is provided in the exhaust passage between the filter and the PM sensor.
In Patent Document 2, there is disclosed an art of providing a first NOx catalyst (a first SCR catalyst) downstream of an oxidation catalyst, providing a second NOx catalyst (a second SCR catalyst) capable of adsorbing more ammonia than the first NOx catalyst downstream of the first NOx catalyst, and further providing urea water supply means upstream of the second NOx catalyst, in a path of exhaust gas.
In Patent Document 3, there is disclosed an art of providing a second catalytic converter that has a larger capacity than a first catalytic converter downstream of the first catalytic converter, and providing an O2 sensor between the first catalytic converter and the second catalytic converter in an exhaust passage.